The present invention relates to an ink jet recording method in which ink droplets are ejected by causing an electrostatic force to act on an ink composition containing at least charged particles containing a colorant and a dispersion medium.
In electrostatic ink jet recording, an ink composition (hereinafter referred to as “ink”) obtained by dispersing charged fine particles containing a colorant (hereinafter referred to as “colorant particles”) in a medium is used, and predetermined voltages are respectively applied to ejection portions of an ink jet head in accordance with image data, whereby the ink is ejected and controlled by utilizing electrostatic forces to record an image corresponding to the image data on a recording medium.
Known as an example of an electrostatic ink jet recording apparatus is an ink jet recording apparatus disclosed in JP 10-138493 A.
FIG. 4 is a schematic view showing an ink jet head of the electrostatic ink jet recording apparatus disclosed in JP 10-138493 A.
The ink jet head 80 includes a head substrate 82, ink guides 84, an insulating substrate 86, control electrodes 88, an electrode substrate 90, a D.C. bias voltage source 92, and a pulse voltage source 94.
Ejection ports (through holes) 96 through which ink is to be ejected are formed so as to extend perfectly through the insulating substrate 86. The head substrate 82 is provided so as to extend in a direction of disposition of the ejection ports 96, and the ink guides 84 are disposed in positions on the head substrate 82 corresponding to the ejection ports 96. Each ink guide 84 extends perfectly through the ejection port 96 so as for its tip portion 84a to project upwardly and beyond the surface of the insulating substrate 86 on an opposite side to the head substrate 82.
The head substrate 82 is disposed at a predetermined distance from the insulating substrate 86. Thus, a passage 98 for ink Q is defined between the head substrate 82 and the insulating substrate 86.
The ink Q containing fine particles (colorant particles) which are charged at the same polarity as that of a voltage applied to the control electrodes 88 is circulated through the ink passage 98 for example from the right-hand side to the left-hand side in FIG. 4, by a circulation mechanism for ink (not shown). Thus, the ink Q is supplied to the corresponding ones of the ejection ports 96.
The control electrode 88 is provided in a ring-like shape on the surface of the insulating substrate 86 on the side of the recording medium P so as to surround the ejection port 96. In addition, the control electrode 88 is connected to the pulse voltage source 94 for generating a pulse voltage in accordance with image data. The pulse voltage source 94 is grounded through the D.C. bias voltage source 92.
In the electrostatic ink jet recording, a recording medium P is preferably held on an insulating layer 91 of the grounded electrode substrate 90 with the recording medium P being charged to a high voltage opposite in polarity to that applied to the control electrode by a charging device utilizing a scorotron charger or the like.
In the electrostatic ink jet recording described above, when no voltage is applied to the control electrode 88, the Coulomb attraction between the bias voltage applied to the counter electrode and the electric charges of the colorant particles in the ink Q, the viscosity of the ink (dispersion medium), the surface tension, the repulsion among the charged particles, the fluid pressure when the ink is supplied, and the like operate in conjunction with one another. Thus, the balance is kept in a meniscus shape as shown in FIG. 4 in which the ink Q slightly rises from the ejection port (nozzle) 96.
In addition, the colorant particles migrate to move to the meniscus surface due to the Coulomb attraction or the like. In other words, the ink Q is concentrated on the meniscus surface.
When the voltage is applied to the control electrode 88 (ejection is valid), the drive voltage is superposed on the bias voltage so that the ink Q is attracted toward the side of the recording medium P (counter electrode) to form a nearly conical shape, i.e., a so-called Taylor cone.
When time elapses after the start of application of the voltage to the control electrode 88, the balance between the Coulomb attraction acting on the colorant particles and the surface tension of the dispersion medium is broken. As a result, there is formed a slender ink liquid column having a diameter of about several microns to several tens of microns which is called a thread. When time further elapses, as disclosed in U.S. Pat. No. 4,314,263 or the like, a tip portion of the thread is divided into small portions, and as a result, droplets of the ink Q are ejected to fly toward the recording medium P.
In the electrostatic ink jet recording, usually, a modulated pulse voltage is applied to the corresponding ones of the control electrodes 88 to turn ON/OFF the corresponding ones of the control electrodes 88 to modulate and eject ink droplets. Thus, the ink droplets are ejected on demand in accordance with an image to be recorded.
JP 2002-370364 A discloses a method of ejecting ink droplets in which the Coulomb force acting on colorant particles in ink and the dielectric polarization force acting on a solvent are controlled to adjust the content of the colorant particles in ink droplets to be ejected thereby achieving compatibility among the recording density, brightness of an image, fixing property, responsivity and the like.
In such electrostatic ink jet recording, when ejection electrodes can be created so as to correspond to ejection portions, independent ink flow paths, partition walls, and the like for separating the ejection portions from each other may be omitted. In this case, a so-called nozzleless structure is obtained, so it becomes possible to achieve cost reduction of the ink jet head and the like and to improve yields. In addition, with the structure described above, even when a problem such as ink clogging has occurred in the ejection portions, it becomes possible to achieve recovery from the trouble through simple processing.
On the other hand, various factors such as properties of an ink composition, properties of a head and a drive voltage affect the electrostatic ink jet recording, which makes the formation of a thread and its division into small portions unstable. The ejection of ink droplets and their landing positions, and the concentration of ink (amount of colorant particles with respect to a dispersion medium) are thus made unstable and an image having the desired image quality cannot be obtained in a consistent manner.
It is possible to improve the recording density, brightness of an image, fixing property, responsivity and the like by adjusting the content of colorant particles in ink droplets to be ejected through control of the Coulomb force acting on the colorant particles and the dielectric polarization force acting on a solvent as in JP 2002-370364 A. However, the formation of a thread and its division into small portions were not stable and the desired image quality could not be attained in a consistent manner.